Underreamers may be used to form an enlarged cavity in a well bore extending through a subterranean formation. The cavity may then be used to collect resources for transport to the surface, as a sump for the collection of well bore formation cuttings and the like or for other suitable subterranean exploration and resource production operations. Additionally, the cavity may be used in well bore drilling operations to provide an enlarged target for constructing intersecting well bores.
One example of an underreamer includes a plurality of cutting blades pivotally coupled to a lower end of a drill pipe. Centrifugal forces caused by rotations of the drill pipe extends the cutting blades outwardly and diametrically opposed to each other. As the cutting blades extend outwardly, the centrifugal forces cause the cutting blades to contact the surrounding formation and cut through the formation. The drill pipe may be rotated until the cutting blades are disposed in a position substantially perpendicular to the drill pipe, at which time the drill pipe may be raised and/or lowered within the formation to form a cylindrical cavity within the formation.
Conventional underreamers, however, suffer several disadvantages. For example, the underreamer described above generally requires high rotational speeds to produce an adequate level of centrifugal force to cause the cutting blades to cut into the formation. An equipment failure occurring during high speed rotation of the above-described underreamer may cause serious harm to operators of the underreamer as well as damage and/or destruction of additional drilling equipment.
Additionally, density variations in the subsurface formation may cause each of the cutting blades to extend outwardly at different rates and/or different positions relative to the drill pipe. The varied positions of the cutting blades relative to the drill pipe may cause an out-of-balance condition of the underreamer, thereby creating undesired vibration and rotational characteristics during cavity formation, as well as an increased likelihood of equipment failure.